1. Field of the Invention
The present invention relates to a semiconductor laser array used as an optical interconnection in an optical communication system or in an information processing system. The present invention also relates to a method for manufacturing such a semiconductor laser array.
2. Description of the Related Art
Recently, an optical interconnection using an optical parallel link system attracts special attention in the field of optical transmission systems between communication channels for computers or the field of ATM exchange systems. A semiconductor laser array or photodetector array for use in such a field is considered as a key device of the optical parallel link system.
Since the semiconductor laser array requires a high-speed operation, NPN transistors which generally operate at a high-speed are used in a drive circuit for driving the semiconductor laser array. For the drive circuit using NPN transistors, a semiconductor laser array having a p-type common electrode is generally used.
FIG. 1 shows a conventional semiconductor laser array including a plurality of semiconductor laser arrays, and FIG. 2 shows the detailed cross-section of the semiconductor laser array of FIG. 1. Referring first to FIG. 1, the semiconductor laser array generally designated at numeral 50 includes a plurality of semiconductor laser devices 52 electrically separated from each other by separation channels 54 to operate independently of each other. The plurality of laser devices 52 are mounted in a junction-up fashion on a common metallic carrier (not shown) with an intervention of a submount 56 disposed between the laser devices and the metallic carrier. An n-side electrode 58 is formed on top of each separate laser device 52, whereas a p-side electrode 60 is formed on the submount 56 as a common electrode layer for the laser devices 52.
Referring next to FIG. 2, the laser active section of each semiconductor laser device comprises a p-type InP (p-InP) cladding layer 64, a strained multiple quantum well (MQW) active layer 66 and an n-InP cladding layer 68 consecutively formed on a p-InP substrate 62. The laser active section is buried by a p-InP burying layer 70, an n-InP blocking layer 72 and a p-InP blocking layer 74, which are consecutively grown on the p-InP substrate 62, at both sides of the laser active section. On the top of the laser active section, there is provided a p-InP contact layer 76, on which a passivation layer 78 is formed over the entire surface except for the n-side electrode 58. The length (L) of the laser active section of each laser device 52 is 200 .mu.m and the pitch (P) of the laser devices 52 in the array is 250 .mu.m, for example. A reflective film having a reflectance of 90% or above is formed on both the facets of the laser device 52.
Before operation, the semiconductor laser array 50 of FIG. 1 is optically connected to a plurality of optical fibers corresponding to the semiconductor laser devices 52. The optical fibers are fixed to the respective laser devices at the locations wherein a maximum coupling efficiency can be obtained. Specifically, the optical fibers are generally fixed to respective portions of a metallic package receiving the semiconductor laser array.
The conventional semiconductor laser array as described above has disadvantages as follows:
It is difficult to epitaxially grow the n-InP blocking layer 72 in the vicinity of the n-InP cladding layer 68 and yet electrically separated from the n-InP cladding layer 68 to obtain a current blocking structure of the laser active section. The current blocking structure is essential to the buried heterojunction (BH) structure of the semiconductor lasers wherein the laser devices 52 operating independently of each other are formed on a p-type substrate from the view point of lower power dissipation and thus have a lower threshold. Moreover, it is difficult to control the process for obtaining stable laser characteristics by, for example, preventing the Zn ions used as a p-type dopant in the p-type substrate from re-diffusing to the laser active section during the epitaxial growth step to vary the laser characteristics.
In the semiconductor laser device, the length of the laser active section should be limited to 200 .mu.m or less for obtaining a low threshold current, and a high reflectance film should be formed on both facets of the laser device to decrease the mirror loss. However, in a laser device having a small length at the laser active section, as is the case of the conventional laser array of FIG. 1, the resin of the high reflectance film flows and attaches to the surface of the electrode during a manufacturing step thereof, thereby impeding a bonding step for the electrode layer.
The thickness of the laser devices generally varies as viewed along the direction of the array in the conventional semiconductor laser array wherein the laser devices are mounted on the carrier with the intervention of the submount. Accordingly, the core center of each optical fiber must be adjusted to the lasing center of the active layers for an efficient optical coupling one by one. If the thickness of the substrate could be controlled in a submicron order, the adjustment might be well effected at a sufficient level. However, it is difficult to attain such an accuracy in the thickness of the substrate even by using a polishing and grinding step for the substrate.